docs: def drop_invalid_columns(feed: "Feed") -> "Feed":
      """
      Drop all DataFrame columns of the given "Feed" that are not
      listed in the GTFS.
      Return the resulting new "Feed".
      """
      feed = feed.copy()
      for table, group in cs.GTFS_REF.groupby("table"):
          f = getattr(feed, table)
          if f is None:
              continue
          valid_columns = group["column"].values
          for col in f.columns:
              if col not in valid_columns:
                  print(f"{table}: dropping invalid column {col}")
                  del f[col]
          setattr(feed, table, f)
  
      return feed